The invention relates to a method of making fibre-based products and their use. One particular application of the invention is the manufacture of a mat like article to hold a catalytic converter inside a can at high temperature.
According to one aspect of the invention there is provided a method of making a fibre-based article, the method comprising:
forming an aqueous slurry containing a major proportion of refractory fibres and a minor proportion of thermoplastic fibres which comprises fibres having two different melting points, one lower than the other;
adding a binder and a flocculating agent;
passing the slurry containing the refractory fibres, thermoplastic fibres, binder and flocculating agent through a perforate screen to deposit a body of wet fibres on the screen;
heating the body to remove the water and to melt the lower melting point thermoplastic fibres to bind the refractory fibres to form a preform; and
thermomoulding the preform then cooling under pressure to form the article.
In another aspect there is provided a dry preform article comprising a major proportion of refractory fibres having a mean length of about 4 cm and a mean diameter of up to about 6 micron, held together by melted thermoplastic fibres and having a substantially uniform density over its area.
Preferably the fibres making up the major proportion are relatively short, say up to 4 cm in length and up to say 6 micron in diameter. The fibres are preferably inorganic fibres such as alumina, mullite, silica, zirconia; graphite: they may be used alone or with other materials such as intumenescent plate materials; particulates; or any materials like these. Combinations may be used. Most preferably the fibres are the ones available under the registered trade mark SAFFIL.
The thermoplastic fibres may be selected from a wide variety of suitable materials having different melting points. The choice is determined by compatability with the other fibres and the temperature at which the fibres melt. The function of this lower melting component is to bond the refractory fibres and hold them together until they are exposed to the temperature at which the thermoplastic fibres are decomposed. Typically suitable fibres are olefins such as polyethylene and polypropylene; polyesters: polyamines. It is preferred to use bicomponent fibres which have having a core and a sheath made of different plastics. Suitable materials are available from different suppliers, e.g. xe2x80x9cTrevira 255xe2x80x9d available from Trevira Fasem, Hartmann Huth, Frankfurt, Germany and xe2x80x9cAdhesion C specialxe2x80x9d fibres available from Fibrevisions. Denmark. The concentration of thermoplastic fibres in the final product may vary; generally we prefer about 10% by weight.
The binder is preferably a latex; the flocculating agent is preferably a polyelectrolyte.
When the deposited body of wet fibres is heated the water is removed and the lower melting point thermoplastic material melts to bind the fibres to form a substantially self-supporting body. The refractory fibres are anchored in the solidified thermoplastic. Because of the way in which the article has been made it has substantially the same consistency and density throughout. The thermoplastic gives the mat integrity making it easier to handle and process.
A further step of thermoulding under compression provides a dense inorganic fibre mat of bulk density suitable for use, e.g. in the monolith in the application.
The process may be batch or continuous.
The article may be used as a support between the inner surface of a catalytic convertor can and the convertor itself. The article may be in the form of mat up to say 8 mm thick which can be used in number of known xe2x80x9ccanningxe2x80x9d techniques. When the convertor is first heated the thermoplastic will burn away to provide a porous resilient structure which holds and supports the catalytic body in place over a prolonged period despite the vibration and fluctuations in temperature. The resulting composition of fibres or fibres (and intrumescent particulates if present) exerts a pressure which supports the delicate convertor during its life. An advantage of the invention is that because it is so self-supporting and resistant to high temperature it is possible to use a mat of the invention in a high temperature environment, for example in a close coupled application, e.g. near an engine exhaust manifold. The mat may be used in vehicles having gasoline, Diesel engines or the like.
In order that the invention may be well understood it will now be described by way of illustration only with reference to the following examples.